The present invention relates to a semiconductor light-emitting device and, more particularly, to a semiconductor light-emitting device made of a nitride compound semiconductor such as GaN, InGaN, AlGaN, or the like.
In recent years, the use of a nitride compound semiconductor represented by GaN has allowed a high-intensity emission of light ranging in color from ultraviolet to blue and green, which had been impossible thus far. As a result, light-emitting devices using nitride compound semiconductors, such as a light-emitting diode (LED) and a semiconductor laser, have been developed vigorously. Since an LED is easier to fabricate and control than a semiconductor laser and longer in lifespan than a fluorescent lamp, an LED using a nitride compound semiconductor is considered to be promising as a light source for illumination.
A description will be given herein below to an example of a conventional nitride compound semiconductor LED. FIG. 16 is a perspective view showing a structure of the conventional nitride compound semiconductor LED disclosed in FIG. 10 of Prior Art Document 1 (Japanese Laid-Open Patent Publication No. 2000-196152).
In the conventional LED, as shown in FIG. 16, a sapphire substrate 101, a GaN buffer layer (not shown), an n-type GaN layer 102, an InGaN active layer 103, and a p-type GaN layer 104 are formed successively by crystal growth and a trench 108 for exposing the n-type GaN layer 102 as the bottom surface thereof has been formed by partly etching away the InGaN active layer 103 and the p-type GaN layer 104. An n-side electrode 106 is provided on the portion of the n-type GaN layer 102 exposed as the bottom surface of the trench 108, while a p-side transparent electrode 105 is provided on the p-type GaN layer 104 and a p-side bonding electrode 107 is provided on a part of the p-side transparent electrode 105.
The following is the operation of the LED. Holes injected through the p-side bonding electrode 107 expand laterally in the p-side transparent electrode 105 to be injected into the InGaN active layer 103 from the p-type GaN layer 104. On the other hand, electrons injected through the n-side electrode 106 are injected into the InGaN active layer 103 from the n-type GaN layer 102. Light emission occurs upon the recombination of the holes with the electrons in the InGaN active layer 103. The light is emitted to the outside of the LED through the p-side transparent electrode 105.
However, such a conventional structure has the problem of low light extraction efficiency. The light extraction efficiency is the ratio of light generated in the active layer and emitted from the LED into an air to all the light generated in the active layer. The cause of the low light extraction efficiency is the refractivity of a semiconductor which is higher than that of the air. As a result, the light from the active layer is totally reflected by the interface between the semiconductor and the air and confined to the inside of the LED. For example, the refractivity of GaN is about 2.45 when the wavelength of the light is 450 nm so that a critical refractive angle at which total reflection occurs is as small as about 23 degrees. That is, light radiated from the active layer at an angle larger than the critical refractive angle in terms of a normal to the interface between the semiconductor and the air is totally reflected by the interface between the semiconductor and the air so that the light emitted from the active layer and extractable to the outside of the LED accounts for only about 4% of all the light emitted from the active layer. Thus, the conventional LED using a nitride compound semiconductor is low in external quantum efficiency (the ratio of light that can be extracted from the LED to currents supplied to the LED) and has the problem of power conversion efficiency (the ratio of a light output that can be produced to all the supplied power) lower than that of a fluorescent lamp.
As a solution to the problem, a technology which forms projections/depressions at the surface of the LED has been proposed, as disclosed in FIG. 5 of Prior Art Document 1. FIG. 17 is a perspective view showing the structure of the conventional nitride compound semiconductor LED disclosed in FIG. 5 of Prior Art Document 1.
In the structure shown in FIG. 17, projections/depressions each of a spherical lens structure have been formed in the p-type GaN layer 104. The structure has the possibility that, even though the angle formed between emitted light and a normal to the interface between the flat portion of the p-type transparent electrode 105 and the air is larger than the critical refractive angle, the angle of incidence of the light becomes smaller than the critical refractive angle if it is incident on the portion provided with the projections/depressions. Accordingly, the probability that the light generated in the active layer is emitted to the outside of the LED without being totally reflected increases and the external quantum efficiency is thereby improved.
However, the technology for improving the light extraction efficiency based on the principle proposed in Prior Art Document 1 is disadvantageous in that, because the angle of incidence of light sensitively varies in response to the configuration of a projecting and depressed surface, the design of the projecting and depressed surface is extremely difficult and characteristics are unstable due to size variations during the fabrication of devices. The technology is also disadvantageous in that, though the projections/depressions to be formed should have a depth of about several micrometers to improve the light extraction efficiency, processing is difficult because of the high etching resistance of a nitride compound semiconductor.